The present invention relates to registration of multiple image exposures formed on a photoreceptor by a plurality of Raster Output Scanning (ROS) systems and, more particularly, to synchronization of multiple ROS's to form registered color images in a single pass system.
In color printing, successive images, each corresponding to a separate and different color, are formed. Each single color separation is transferred to a copy sheet in superimposed registration with any prior single color image. The registered relationship of the single color images creates a multi-layered image on the copy sheet which forms a composite color image. In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. This laser beam is modulated in conformance with the image information. The modulated beam is transmitted through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoconductive member in a fast scan (i.e. line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. Laser printers may operate in either a single pass or a multiple pass system.
In a single pass, process color xerographic printing system, three ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station may be added if black images are to be created as well. In a multiple pass system, each image area on the photoreceptor surface must make at least three passes relative to the transverse scan line formed by the modulated laser beam generated by a ROS system. With either system, each image must be registered to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (slow scan or skew registration) and in direction perpendicular to the process registration (referred to as fast scan or transverse registration).
Any relative displacement of the color images is very noticeable. Different schemes have been proposed to solve this color misregistration. Errors in the image positioning can arise from changes in the velocity and phase of the rotating polygon mirrors in the ROS stations or units. Any motor polygon assembly (MPA) to MPA operational differences are eventually cascaded as color to color misregistration.
Synchronization of the multiple ROS's will minimize the misregistration of multiple image exposures formed on a photoreceptor by a plurality of Raster Output Scanning (ROS) systems in a single pass system.
It is an object of the present invention to register the color images by synchronizing the multiple ROS of the printing system.